The present invention relates to a transceiver for use in wireless communication. In particular, but not exclusively, the present invention relates to a transceiver that has a dual mode of operation where in the first mode of operation the signals received and the signals transmitted by the transceiver are in different frequency ranges and in the second mode of operation the received and transmitted signals are in the same frequency range.
In most cellular telecommunication networks, mobile stations in a cell associated with a base transceiver station use a first frequency range to transmit signals to the base transceiver station. The base transceiver station uses a second, different, frequency range to transmit signals to the mobile stations in the cell associated with that base transceiver station. This is known as the frequency division duplex (FDD) mode of operation and has been used in analogue cellular telecommunication systems as well as in the current digital cellular telecommunications systems such as GSM.
Reference is made to FIG. 10a which illustrates the principle of FDD operation in a frequency division multiple access system. The first frequency range F1 is used to transmit signals from the mobile stations to the base station and the second frequency range F2 is used to transmit signals from the base station to the mobile stations. In practice each of the frequency ranges F1 and F2 are divided into a plurality of smaller frequency ranges f1 and f2 respectively. Thus a mobile station in a particular cell will be allocated one of the smaller frequency ranges f1 to communicate with the base station associated with the cell in which the mobile station is located. Likewise, the base station will be allocated one of the smaller frequency ranges f2 to communicate with the mobile station.
In a system such as the GSM system which uses time division multiple access (TDMA), each smaller frequency range is divided into a plurality of frames 100, one of which is shown in FIG. 10d. Each frame 100 comprises a plurality of time slots 102. The base station will be allocated a particular time slot 102 in successive frames to communicate with a given mobile station in one of the smaller frequency ranges f2. Likewise the mobile station will be allocated a particular time slot in successive frames to communicate with the base station in one of the smaller frequency ranges f1.
It has also been proposed that in some systems the base transceiver station can also use the same frequency range to communicate with a mobile station as the mobile station uses to communicate with the base station. This is known as the time division duplex (TDD) mode and is for example used in the DECT system. The frame and slot structure illustrated in FIG. 10d is also used. Thus in the TDD mode certain of the time slots in each frame will be allocated for use by the mobile stations to transmit signals to the base transceiver station. The remaining slots in each frame will be used by the base transceiver station to send signals to the mobile stations.
It has been proposed to have dual mode mobile stations which, for example can use both GSM and DECT modes of operation.
In the systems described hereinbefore which use time division multiple access|, a mobile station will not receive and transmit signals at the same time. Accordingly, no consideration needs to be given to the isolation of the signals received and transmitted by the mobile station.
Another wireless communication access method is the code division multiple access (CDMA) method that also uses a first frequency range F1 for signals transmitted from mobile stations to the base transceiver station and a second frequency range F2 for signals transmitted from the base transceiver station to the mobile stations. The CDMA system like the above described FDMA/TDMA system in that, as shown in FIG. 10b, has the first and second frequency ranges divided into smaller frequency ranges. However the same frame and time slot structure is not used. Instead each signal is transmitted in one of the smaller frequency ranges and signals in the same smaller frequency range are distinguished by the spreading codes applied to the signals. In the CDMA method, the mobile station may receive and transmit signals at the same time. Accordingly, mobile stations which use the CDMA method have a duplex filter connected to the antenna. The duplex filter needs to have sufficient isolation to prevent the received signal from interfering with the signal to be transmitted and vice versa.
Three different transceivers have been considered by the inventors for implementation of a mobile station which can operate in a FDD mode and a TDD mode and which can receive and transmit signals at the same time in the FDD mode of operation. In both modes of operation CDMA, TDMA or any other appropriate HIS method is used. It should be noted that these three transceivers, which are shown in FIGS. 1 to 3, do not constitute part of the state of the art.
A first one of these transceivers is shown in FIG. 1 and is now described in detail. An antenna 2 is arranged to receive signals and also to transmit signals. The antenna 2 is arranged to transmit signals in the frequency range F1 and to receive signals in the frequency ranges F2 and F1. The antenna 2 is connected to a switch 4 which selectively connects either a duplex filter 6 or a bandpass filter 8 to the antenna 2. The position of the switch 4 is dependent on the mode of operation and will be discussed in more detail hereinafter. The duplex filter 6 comprises a receive filter portion 6a which is tuned to frequency range F2 and a transmit filter portion 6b which is tuned to frequency range F1. The bandpass filter 8 is tuned to frequency F1. A switch 10 selectively connects the filter 8 or the receive filter portion 6a of the duplex filter 6 to a low noise amplifier 12. The output of the low noise amplifier 12 is connected to a mixer 14 which mixes the received signal with a signal from a high frequency synthesizer 16. The high frequency synthesizer 16 acts as a local oscillator. The output of the mixer 14 is at an intermediate frequency which is usually lower than the frequency of the signal which is received by the antenna 2. Other parts of the transceiver are known to those skilled in the art and will not be described.
In a similar manner, a signal to be transmitted is input at an intermediate frequency to a second mixer 18. The second mixer 18 also has an input from a second high frequency synthesizer 20 which again acts as a local oscillator. The output of the second mixer 18 represents the signal to be transmitted at the radio frequency and is in the frequency range F1. The radio frequency is the frequency at which the signal is transmitted across the channel to the base transceiver station. The output of the second mixer 18 is input to a power amplifier 22 which amplifies the signal. The output of the power amplifier 22 is received by the transmit portion 6b of the duplex filter 6 where the signal is filtered to remove undesired signal components and noise outside the frequency range F1.
The transceiver shown in FIG. 1 has two modes of operation. In the FDD mode of operation, the switch 4 connects the antenna 2 to the receive filter portion 6a of the duplex filter 6. The antenna 2 is not connected to the filter 8. The second switch 10 connects the receive portion 6a of the duplex filter 6 to the low noise amplifier 12. Thus, the signals received by the antenna 2 pass through the receive filter portion 6a of the duplex filter 6 to the low noise amplifier 12. From the low noise amplifier 12 the received signals are passed to the mixer 14.
In the FDD mode, the signals to be transmitted pass from the second mixer 18 to the power amplifier 22. The signals to be transmitted are then filtered by the transmit portion 6b of the duplex filter 6 and output to the antenna 2. The antenna 2 can, in this mode of operation, receive and transmit signals at the same time.
In the TDD mode of operation, the switches 4 and 10 are in the position shown in FIG. 1. The antenna 2 is connected to the bandpass filter 8 which in turn is connected via switch 10 to the low noise amplifier 12. The duplex filter 6 is not connected to the antenna 2 or to the low noise amplifier 12. In the TDD mode, the antenna 2 does not transmit and receive signals at the same time. The positions of the switch 4 will need to be altered if a signal is to be transmitted. Both the transmitted and the received signals will be in the frequency range F1.
This transceiver has the disadvantage that the two switches 4 and 10 both need to have good isolation. Typically, the total order of isolation needed for the switches is between 50 and 60 decibels. This isolation is required to prevent the signal which is transmitted in the FDD mode of operation from being partially fed through the bandpass filter 8 to the low noise amplifier 12, even when the switch 4 connects the antenna 2 to the duplex filter 6. If the signal to be transmitted reaches the low noise amplifier 12, because the first and second switches 4 and 10 do not have sufficient isolation to prevent this occurring, there could be interference between the signal which is to be transmitted in the frequency range F1 and the signal which should be received in the frequency range F2. If the part of the signal which is to be transmitted which reaches the low noise amplifier 12 has a much higher power level than the signal which should be received, the signal which should be received may be blocked. The information carried by the signal which should be received may therefore be lost or garbled.
This problem is compounded by the fact that the transmitted signal will have a much greater strength than the received signal. This problem could be avoided if the two switches 4 and 10 have sufficient isolation to prevent the feedback of the transmitted signal. Radio frequency switches having the required isolation to deal with this problem are both expensive and difficult to implement in practice.
Another problem associated with the transceiver shown in FIG. 1 is that the first switch 4 needs to have good linearity so as to avoid distorting the signal which is to be transmitted by the antenna 2. This further increases the costs associated with the first switch 4 as well as the difficulties associated with the implementation of that switch. In particular, it is difficult to implement a switch which has good isolation as well as good linearity.
FIG. 2 shows a second dual transceiver which has a FDD mode of operation and a TDD mode of operation. The FDD mode of operation may use CDMA whilst the TDD mode of operation may use TDMA or a CDMA/TDMA hybrid mode of operation. Those components which are the same as those of FIG. 1 are marked with the same reference numerals and will not be described again. The primary difference between the arrangement of FIG. 2 and that of FIG. 1 is that the duplex filter 6 comprises a tunable receive filter portion 6xe2x80x2a along with the transmit filter portion 6b. The transmit filter portion 6b is the same as that of the duplex filter 6 of FIG. 1.
In the FDD mode of operation, the receive filter portion 6xe2x80x2a is tuned to the frequency range F2. The transceiver is thus able to receive signals with a frequency in the range F2 and to transmit signals with a frequency in the range F1. In the TDD mode of operation, the receive filter portion 6xe2x80x2a is tuned to frequency F1. Thus, in the TDD mode of operation, signals are received and transmitted at frequency F1, although not at the same time. However, it is again difficult and expensive to implement a tunable duplex filter which has sufficient isolation to prevent at least a part of the signal to be transmitted, in the FDD mode, from reaching the low noise amplifier 12. For example, 50 to 60 decibels of isolation may be required. Isolation is an inherent problem with standard duplex filters but this problem is made worse if the duplex filter has a tunable filter portion.
A third transceiver is shown in FIG. 3. Those components which are the same as those shown in FIG. 1 are referenced by the same reference numerals and these components will not be described further. The antenna 2 is connected to a duplex filter 6 which has a receive filter portion 6a tuned to frequency range F2 and a transmit filter portion 6b which is tuned to frequency range F1. One switch 23 is provided. In the FDD mode of operation, the switch 23 is in the position shown in FIG. 3 and connects the receive filter portion 6a to the low noise amplifier 12. The received signals pass through the receive filter portion 6a, via switch 23, to the low noise amplifier 12. The signals to be transmitted pass from the power amplifier 22 to the transmit filter portion 6b of the duplex filter 6.
In the TDD mode of operation, the switch 23 connects the transmit filter portion 6b of the duplex filter 6 to the low noise amplifier 12 so that the received signals pass through the transmit filter portion 6b to the low noise amplifier 12. The disadvantage with this arrangement is that the switch 23 has to deal with radio frequency signals and provide good isolation, typically 50 to 60 decibels. In the FDD mode of operation, if the isolation provided by switch 23 is not sufficient, interference between the signal which is to be transmitted and the received signal may occur. This is a similar problem to that which occurs with the arrangement shown in FIG. 1. Additionally, the switch 23 also requires good linearity for the same reasons as outlined in relation to FIG. 1. It is difficult and expensive to implement a switch which has both the required linearity and the required isolation.
In known CDMA and TDMA systems, to improve performance, the maximum ratio combining space diversity technique is sometimes used. A transceiver using this technique is shown in FIG. 4. This transceiver is used only in relation to single mode systems (FDD or TDD) and use of this transceiver with respect to dual mode mobile stations has not been contemplated. The components which are the same as those shown in FIGS. 1 to 3 are referred to by the same reference numerals. The arrangement shown in FIG. 4 comprises an antenna 2 connected to a duplex filter 6 having a receive filter portion 6a tuned to frequency range F2 and a transmit filter portion 6b tuned to frequency range F1. The output of the receive filter portion 6a of the duplex filter 6 is input to a low noise amplifier 12, the output of which is connected to a first mixer 14. The first mixer 14 receives an input from a high frequency synthesizer 16 so that the output of the mixer 14 represents the received signal at an intermediate frequency.
A path for the signal to be transmitted consists of a second a mixer 18 which also has an input from the first high frequency synthesizer 16, a power amplifier 22 and the transmit filter portion 6b of the duplex filter 6.
The transceiver of FIG. 4 comprises a second antenna 30 which is physically spaced apart from the first antenna 2. The second antenna 30 is used only to receive signals and is connected to a receive filter 32 which is tuned to frequency range F2. The output of the filter 32 is connected to a second low noise amplifier 34, the output of which is connected to a third mixer 36. The third mixer 36 has an input from a second high frequency synthesizer 38 so that the output of the third mixer 36 represents the received signal but at an intermediate frequency.
The second antenna 30 is used as a diversity receiver. For example, if the signal from a base station is not strongly received by antenna 2, it may be strongly received by antenna 30 and vice versa. This is due to the signals received by the first and second antenna following different paths from the base station to the mobile station. The signals received by antenna 2 and antenna 30 may be coherently combined using the maximum ratio combining technique, which gives a better performance, or, alternatively, the strongest signal selected. The combining of the signals or the selection of the strongest signal is carried out by components of the transceiver which are not shown in FIG. 4.
It is an aim of embodiments of the present invention to provide a transceiver which is able to support two modes of operation and which at least reduces the problems of the arrangements discussed hereinbefore. Preferably the two modes of operation are the TDD mode of operation and the FDD mode of operation.
According to one aspect of the present invention, there is provided a transceiver for use in wireless communications comprising: a first antenna and first filter means coupled ho thereto to transmit signals in a first frequency range; and a second antenna and second filter means coupled thereto to receive signals in the first frequency range or a second different frequency, in dependence on a mode of operation of the transceiver, whereby in a first mode of operation of the transceiver, the transceiver is arranged to transmit signals in the first frequency range via the first antenna and to receive signals in the second frequency range via the second antenna and in a second mode of operation, the transceiver is arranged to receive signals in the first frequency range via the second antenna.
As the signals are received and transmitted by different antennae the isolation required is reduced. This is because there will be a coupling loss between the antennae which then reduces the amount of isolation required. This coupling loss arises because the first and second antennae are physically spaced apart from one another.
Embodiments of the present invention may be used with any system which uses different frequency ranges for uplink and downlink communications.
The first mode of operation may be the FDD mode of operation whilst the second mode of operation may be the TDD mode of operation.
Whilst all signals may only be received by the second antenna and all signals only be transmitted by the first antenna, it is preferred that the first antenna and first filter means be arranged to receive, in the first mode of operation, signals in the second frequency range. It is preferable in this arrangement that signals only be transmitted via the first antenna.
Preferably, processing means process signals received by the first and second antennae in the first mode of operation to provide space diversity so that performance may be improved. The signals can be combined coherently or alternatively, the stronger of the signals received by the first and second antennae can be selected. The combining may be coherent combining such as maximum ratio combining. The signals which are received by the transceiver will typically travel via a number of different paths. This is known as the multipath effect. Any problems caused by a signal being attenuated due to the paths travelled by that signal can be reduced in that a signal travelling via an alternative paths which reaches the other antenna may be strong enough. Thus, embodiments of the present invention are able to provide both space diversity and also a transceiver which is able to operate in the FDD and TDD mode of operation but which does not suffer from the disadvantages of the arrangements discussed hereinbefore.
Preferably, the first filter means comprises a duplex filter having a first portion tuned to the first frequency range and a second portion tuned to the second frequency range whereby in use the signals to be transmitted are filtered by the first portion and the received signals are filtered by said second portion.
The second filter means may comprise a first filter tuned to the first frequency range and a second filter tuned to said second frequency range. Preferably switch means are arranged to couple the second antenna to the second filter in the first mode of operation and to couple the second antenna to the first filter in the second mode of operation. Due to the coupling loss between the first and second antenna, the switch means does not need to provide the same degree of isolation as required in the arrangements discussed hereinbefore. Accordingly, the switch means can be more easily and more cheaply implemented.
Alternatively, the second filter means may comprise a tunable filter which, in the first mode of operation is tuned to the second frequency range and in the first mode of operation is tuned to the first frequency range. Again, due to the coupling loss between the antennae, the degree of isolation which needs to be provided by the tunable filter is reduced and this tunable filter can be more easily implemented than in the known arrangement shown in FIG. 2.
Preferably, the output of the first and second filter means are coupled to respective amplifier means and mixer means. Preferably, in the first mode of operation, the transceiver is arranged to receive and transmit signals at the same time. However, in the second mode of operation, the transceiver may be arranged so that it does not receive and transmit signals at the same time. In the second mode of operation, the transceiver is preferably arranged to transmit signals via the first antenna in the first frequency range, the transceiver not transmitting and receiving signals at the same time in the second mode of operation.
The first frequency range may have a first part and a second part, whereby in the first mode of operation, said first antenna is arranged to transmit signals in the first part of the first frequency range and in the second mode of operation, said second antenna is arranged to receive signals in the second part of the first frequency range. Preferably the first antenna is arranged in the second mode of operation to transmit signals in the second part of the first frequency range.
Preferably, the transceiver is arranged to receive and transmit signals in the code division multiple access format, the time division multiple access format or any other suitable format. Thus, embodiments of the present invention can be used with any other spread spectrum or non-spread spectrum access technique. A mobile station preferably incorporates a transceiver such as described hereinbefore.